This invention relates to a communication system and a transmission power control method and, in particular, to an outer-loop transmission power control method in a CDMA (Code Division Multiple Access) communication system.
In a CDMA communication system, excessive transmission power to a receiving side results in an increase in interference with other receiving parties and a decrease in line capacity. It is therefore necessary to always carry out transmission with transmission power optimum for the receiving side. To this end, inner-loop transmission power control and outer-loop transmission power control are carried out in combination in the receiving side. In the inner-loop transmission power control, power control is carried out at a high speed by transmitting a TPC (Transmit Power Control) bit per slot from the receiving side to a transmitting side so that a reception SIR (Signal-to-Interference power Ratio) is equal to a target SIR. In the outer-loop transmission power control, the target SIR in the inner-loop transmission power control is controlled in the receiving side so as to keep a constant error rate, such as FER (frame error rate) and a BLER (block error rate), representative of a line quality.
However, in a service, such as packet communication, in which transmission and reception of data are not continuously carried out, CRC (cyclic redundancy check) results for the data can not always be measured. This leaves a time period during which the line quality can not be measured and the outer-loop transmission power control can not be carried out.
In order to avoid the above-mentioned problem, CRC bits alone are continuously transmitted from the transmitting side to a user even if there is no user data. In this manner, quality measurement is possible. In this case, however, redundant bits (that is, the CRC bits) are transmitted exclusively for the purpose of quality measurement for the outer-loop transmission power control. For other users, an interference is increased. As a result, the line capacity is disadvantageously decreased.
On the other hand, in the inner-loop transmission power control, a SIR of an object channel must be measured. Therefore, pilot bits, which are generally known symbols, must simultaneously be used. Accordingly, even if there is no user data, the pilot bits, the TPC bits, and the CRC bits must be transmitted.
This is because the pilot bits for use in SIR measurement and the CRC bits for use in measurement of the line quality separately exist.
Now, referring to FIGS. 1 through 4, description will be made of an example of transmitting and receiving operations in a related communication system of the type in case where there is no user data.
Judgment of presence or absence of the user data can be realized by using control bits indicative of presence or absence of data and a data length, in addition to the pilot bits. However, description thereof is omitted herein and the operation when absence of the user data is judged will only be described. Description of the TPC bits for the inner-loop transmission power control is also omitted herein. A bit length, an interleaving length, and the number of pilot bits are given specific values In the following description but, not being limited thereto, may have any values.
Referring to FIGS. 1 and 2, an operation at a transmitting side is as follows.
(1) There is no user data to be transmitted.
(2) CRC parity bits alone are added. By way of example, the CRC parity bits of 8 bits all of which are “0” bits are added.
(3) The CRC parity bits are coded. By way of example, convolution coding at a coding rate of 1/0.2 is carried out. In the illustrated example, a data sequence of 8 bits is coded into 32 bits. In the illustrated example, in case where 8 bits all of which are “0” bits are coded, 32 bits obtained after coding are all “0” bits.
(4) In order to match a frame length, DTX (Discontinuous Transmission) bits are inserted. The DTX bits represent that the transmission power is adjusted to “0” upon transmission. In the illustrated example, the DTX bits of 32 bits are inserted.
(5) An interleaving operation is carried out. In the illustrated example, the interleaving operation when C=8 is carried out. As illustrated in the figures, by writing in a horizontal direction and reading in a vertical direction, the order of a bit sequence is changed.
(6) After the interleaving operation, pilot bits of 4 bits are inserted per 8 bits. In the illustrated example, a repetitive pattern “1010” is shown. However, any other known pattern may be used.
(7) In order to carry out modulation such as phase modulation, those bits are converted into symbols. In the illustrated example, “0” bits are converted into symbols “+A” and “1” bits are converted into symbols “−A”. Herein, “A” represents an amplitude. The DTX bits are converted into symbols “0” so that the amplitude is “0”.
After the above-mentioned operation upon transmission bits, modulation such as phase modulation is carried out to obtain a modulated symbol sequence. Then, the modulates symbol sequence is transmitted from a base station as a radio wave. Practically, spreading is carried out in CDMA communication. However, description thereof is omitted herein.
Next referring to FIGS. 3 and 4, an operation at a receiving side will be described.
(8) By demodulating the radio wave from the base station as a phase-modulated wave, a reception symbol sequence is obtained.
(9) In the reception symbol sequence, a pilot symbol part is used in SIR measurement and a demodulating operation. However, the pilot symbol part is not used in a data decoding operation subsequently carried out and, therefore, is removed.
(10) By carrying out an operation reverse to that described in (5), a deinterleaving operation is carried out to recover the order of the data sequence into an initial order.
(11) The DTX bits are deleted. In the illustrated example, the bit length is assumed to be known. Actually, however, the bit length can be calculated by control bits representative of presence or absence of data and a data length.
(12) The convolution-coded bit sequence is decoded. Generally, the Viterbi algorithm is known in which soft-decision decoding is carried out.
If 8 bit data comprising “0” bits alone are recovered by the above-mentioned procedure, the result of CRC judgment is normal (OK). If at least one bit is in error, the result of CRC judgment is abnormal (NG).
Japanese Patent Application Publication (JP-A) No. 2003-32184 discloses another control of a target SIR following variation in propagation environment in an outer-loop control.
In a service, such as packet communication, in which data transmission and reception are not continuously carried out, it is a general practice that the CRC bits are continuously transmitted to carry out quality measurement. In this case, however, redundant bits (that is, the CRC bits) are transmitted exclusively for the purpose of quality measurement for the outer-loop transmission power control. For other users, an interference is increased. As a result, the line capacity is disadvantageously decreased.